1. Field of the Invention
The present invention relates to a semiconductor device having an SOI (silicon-on-insulator) structure with a silicon layer formed on an insulator layer, and to a method of manufacturing it.
2. Description of Related Art
FIG. 8 shows a semiconductor device having a conventional SOI structure. As illustrated therein, an insulator layer 102 is formed on a semiconductor substrate 101, and a silicon layer 103 is formed on the insulator layer 102 to give an SOI structure.
In the semiconductor element region comprising the silicon layer 103, formed are source/drain regions 103a through doping with an impurity such as phosphorus, arsenic or the like, or boron or the like, and, in the area between the source/drain regions 103a, formed is a gate electrode 107 on the silicon layer 103 via a gate oxide film 106 therebetween to give a MOSFET.
A trench 104 is formed at the element-isolation area of the silicon layer 103, and an insulating film 105 is formed within the inner wall of the trench 104 to give an element-isolation region, serving to isolate semiconductor elements from each other. As in FIG. 8, the bottom face of the silicon layer 103 makes an acute angle with the side of the element-isolation region (insulating film 105) adjacent thereto.
An interlayer insulating film 108 is formed on the SOI substrate having the MOSFET thereon, and a conductive layer 109 is formed on the interlayer insulating film 108. Contact holes for enabling electric connection between the conductive layer 109 and the source/drain regions 103a formed in the silicon layer 103 are formed through the interlayer insulating film 108 and filled with a conductor, via which the conductive layer 109 is electrically connected with the source/drain regions 103a. 
In the element-isolation region of the semiconductor device having the SOI structure of that type, the bottom face of the silicon layer 103 makes an acute angle with the side of the element-isolation region (insulating film 105) adjacent thereto. In that condition, therefore, when the volume of the element-isolation region is varied through heat treatment to be effected after the formation of the element-isolation region, for example, through annealing to be effected after the formation of the oxide film 105 in the trench 104, or through heat treatment to be effected in forming the gate oxide film 106 after the formation of the element-isolation region, the volume variation shall make the silicon layer 103 have large stress at the acute-angled corners of its bottom.
A technique for relaxing the large stress at the acute-angled corners of the bottom of the silicon layer 103 is disclosed, for example, in Unexamined Japanese Patent Publication No. (HEI)6-216230. FIG. 9 shows an SOI structure for a semiconductor device illustrated in Unexamined Japanese Patent Publication No. (HEI)6-216230. As illustrated, the trench-shaped insulator of constructing an element-isolation region is so formed that its width in the cross section is continuously increased in the downward direction, in order that the bottom of the silicon layer does not make an angle with the element-isolation region adjacent thereto. Therefore, being different from that of FIG. 8, the semiconductor device of FIG. 9 has no angle that may produce large stress, and the bottom of the silicon layer in FIG. 9 is prevented from having any large stress. In FIG. 9, numeral references are the same as those in FIG. 8, provided that the element-isolation region 105 is formed of an isolating wall 105b in the trench 104 and a polysilicon layer 105a embedded therein.
In the semiconductor device noted above, however, the interface of the silicon layer adjacent to the insulator is formed to be convex toward the insulator. In this, therefore, when the stress resulting from the volume variation in the element-isolation region runs toward the silicon layer, it is concentrated in some parts in the silicon layer, as will be mentioned below. The problem caused by the stress concentration is that some lattice defects are formed in those parts with the stress concentrated.
The reason for the stress concentration is described with reference to FIG. 10. As illustrated, in the semiconductor device of FIG. 10, the interface between the silicon layer and the insulator (trench) is so formed that, reaching the insulating layer, it is curved toward the silicon layer but not toward the element-isolation region. In this, therefore, when the stress resulting from the volume variation in the element-isolation region runs toward the silicon layer, as indicated by the arrows in FIG. 10, a plurality of stress components running in that direction shall be concentrated in the part as designated by xe2x80x9cXxe2x80x9d therein. As a result, some lattice defects are formed in that part of the silicon layer.
Meanwhile a semiconductor device having a trench-shaped insulator of which the width is continuously decreased in the downward direction, is disclosed in IEDM (International Electron Devices Meeting) Technical Digest, 1997, p.591. However, the semiconductor device disclosed has a large depression at the surface of the trench-shaped insulator near the semiconductor element region adjacent thereto. Thus, in the case where a gate electrode extends over the trench-shaped insulator as well as the semiconductor element region, a portion of the gate electrode will be embedded in the depression of the insulator so that the distance between the semiconductor element region and the portion of the gate electrode located on the insulator will be shortened in comparison with the case of no depression. With this structure, when a transistor controllable with such a gate electrode is formed in the semiconductor element region, it will likely be influenced by an electric field from the portion of the gate electrode embedded in the depression of the adjacent insulator, that is, an electric field will be concentrated in the semiconductor element region near the depression. As a result, a reverse narrow channel effect decreasing a threshold voltage occurs and a parasitic MOSFET tends to be generated in the semiconductor element region near the depression. The concentration of an electric field may also cause a deterioration of the semiconductor element region, such as silicidation of a contact portion of the source/drain region of the transistor.
On the other side, Unexamined Japanese Patent Publication No. (HEI)9-8118 discloses a process for forming a trench-shaped insulator without a depression at the surface near the semiconductor element region adjacent thereto, but the width of the trench-shaped insulator formed is constant in the downward direction.
The object of the invention is to provide a semiconductor device in which the stress from the volume variation in the element-isolation region to the silicon layer is relaxed to thereby protect the silicon layer from having lattice defects therein and in which the surface of the trench-shaped insulator in the element-isolation region does not have a depression near the semiconductor element region, and to provide a method of manufacturing it.
The semiconductor device of the invention comprises an SOI substrate with a silicon layer formed on an insulating layer, and a semiconductor element region and an element-isolation region formed in the silicon layer, wherein; the element-isolation region is of a trench-shaped insulator formed adjacent to the semiconductor element region, and the trench-shaped insulator is provided with a portion which is adjacent to the semiconductor element region, of which the width is continuously decreased in the downward direction, and of which the surface is planarized near the semiconductor element region. In the device, even when the volume variation in the element-isolation region produces some stress running toward the silicon layer, the stress to the silicon layer could be well relaxed. In addition, because the surface of the trench-shaped insulator in the element-isolation region is made flat near the semiconductor element region, the concentration of an electric field in the semiconductor element region does not occur when a gate electrode is extended over the trench-shaped insulator and the semiconductor element region.
One method of manufacturing the semiconductor device of the invention comprises, forming, on an SOI substrate having a silicon layer formed on an insulating layer, a protective film having an unprotected portion of a predetermined pattern, forming an insulating film over the SOI substrate with the protective film, followed by etching the insulating film to leave an insulating side wall at the side of the unprotected portion of the protective film, anisotropically etching the silicon layer via the protective film and the insulating side wall both acting as a mask to form a trench in the silicon layer, and subjecting the silicon layer with the trench to a heat treatment to oxidize the side portion of the trench to thereby form a trench-shaped insulator having a width continuously decreased in the downward direction. In the method, the curved profile at the interface is controlled by varying the condition for oxidizing the side part of silicon.
Regarding the temperature for the heat treatment in the method, if it is too low, suitable oxidation could not be attained at such low temperatures. On the contrary, if it is too high, the oxide film could be formed even at such high temperatures. In this case, however, the interface could not be curved but is linear. For these reasons, it is desirable that the temperature for the heat treatment falls between 750xc2x0 C. and 900xc2x0 C.
Another method of manufacturing the semiconductor device of the invention comprises, forming, on an SOI structure having a silicon layer formed on an insulating layer, a protective film having an unprotected portion of a predetermined pattern, forming an insulating film over the SOI substrate with the protective film, followed by etching the insulating film to leave an insulating side wall at the side of the unprotected portion of the protective film, anisotropically etching the silicon layer via the protective film and the insulating side wall both acting as a mask to form a trench in the silicon layer, and isotropically etching the side portion of the trench to partially remove a portion of the silicon layer underlying the insulating side wall to obtain a trench having a width continuously decreased in the downward direction, and forming an insulator in the trench. In the method, the curved profile at the interface is controlled by varying the etching condition.